---
topic: building it
status: accepted
date: 2026-09-24
deciders: jochen
reconstructed: false
---

# 107. Persistent data is a directory bind, never a named volume

## Context

Measured 2026-09-24, mid-migration: four modules mount a named Docker volume for real state —
`mesh-store` (every database the mesh holds), `mesh-broker` (its data and TLS material),
`mesh-registry` (every image), and `searxng`'s cache. Every other module in the catalogue —
more than forty — mounts a host directory, `/var/lib/<module>/...`.

The predecessor did not make this choice. HAL's own `postgres` bound `./db-data`, and its
`lavinmq` bound `./data` — directories, both. The mesh's adoption of them
([`a63ef3d`](https://git.novox.be/novox/mesh-catalog/commit/a63ef3d), "the postgres module
adopts mesh-store instead of raising its own", 2026-09-16) introduced the named volume; three
other modules followed the same shape since. Full account of what was found and fixed:
[issue 115](../04-ISSUES/115-a-named-docker-volume-is-invisible-and-one-flag-from-gone/00-report.md).

## The two guarantees are not the same guarantee

A named volume and a host directory both survive **ordinary** container recreation — a rebuild,
a `take`, the `docker rm -f` and push this migration already uses routinely. Neither loses data
to that. That was never the question.

What they do not both survive:

- **`docker rm -fv`, `docker volume rm`, `docker system prune --volumes`** all target a named
  volume specifically. The first is one character from the command this migration's own rules
  already call for after every address change. A host directory has no equivalent command that
  destroys it by accident — removing it is always a deliberate `rm -rf` on a path someone typed.
- **Visibility.** Every tool this migration has used all night to find and verify data —
  `ls`, `find`, `grep`, a backup job — reaches a host directory for free. A named volume requires
  knowing to ask Docker (`docker volume inspect`) before its bytes, at
  `/var/lib/docker/volumes/<name>/_data`, are reachable at all.

## Decision

**A container mount holding data that must survive is a host directory bind. A named volume is
permitted only for data that is disposable if lost** — a cache, a scratch space, something the
module rebuilds on next start without consequence. `searxng`'s `valkey` cache is close to this
line and was converted anyway, for consistency and because it costs nothing to.

Ownership is the one thing a host directory does not get for free that a named volume does:
Docker initialises a fresh named volume's ownership to what the container's first process needs;
a host directory is whatever created it. A directory made for this purpose must be given the
image's expected UID before the container using it starts — read from the running instance being
replaced when one exists, rather than guessed.

## Consequences

- The four modules were converted: `distribution`, `lavinmq`, `postgres`, `searxng`. Data copied
  and verified byte-for-byte before each manifest changed; `mesh-store` stopped cleanly first, so
  its copy is crash-consistent rather than a live read of a running postgres.
- **The ownership gap above was not theoretical — it is what happened.** The new directories
  were created by the operator's tooling running as root; `mesh-store` crash-looped on
  `mkdir: ... Permission denied` until its directory's ownership was set to match what the
  original volume already had. Worth a check at `module add` time — nothing catches this today
  beyond the container failing to start.
- Old named volumes were not deleted. They remain the rollback path until confidence in the new
  mounts is established over time, not one clean start.

## References

- [Issue 115](../04-ISSUES/115-a-named-docker-volume-is-invisible-and-one-flag-from-gone/00-report.md)
- `mesh-catalog` PR #54